# Reference for the multivariate Leibniz rule of many factors

I'm looking for a reference (a book/article) with a formula to $$\frac{ \partial ^ k }{ \partial x_1^{k_1} ... \partial x_n^{k_n} } f_1(x) ... f_m(x) ,$$ where $$k=k_1+...+k_n$$, $$x=(x_1,...,x_n)\in\mathbb{R}^n$$, $$f_i\colon\mathbb{R}^n\to\mathbb{R}$$.